1. Technical Field
The present invention generally relates to a solvent extraction method for preparing a salt of a sulfurized alkyl-substituted hydroxyaromatic composition having a reduced content of unsulfurized alkyl-substituted hydroxyaromatic compound and its unsulfurized metal salt.
2. Description of the Related Art
The lubricant additive industry generally uses alkyl phenols (e.g., tetrapropenyl phenol, TPP) to prepare detergents comprising sulfurized metal alkyl phenates. Metal salts of sulfurized alkylphenols are useful lubricating oil additives which impart detergency and dispersancy properties to the lubricating oil composition for marine, automotive, railroad and air-cooled engines as well as providing for an alkalinity reserve in the oil. Alkalinity reserve is necessary in order to neutralize acids generated during engine operation. Without this alkalinity reserve, the acids so generated would result in harmful engine corrosion. However, there may be some unreacted alkyl phenols such as tetrapropenyl phenol present in the sulfurized metal alkyl phenate as well as in lubricating oils containing one or more of the sulfurized metal alkyl phenates.
A recent reproductive toxicity study in rats sponsored by the Petroleum Additives Panel of the American Chemistry Council shows that free or unreacted TPP may cause adverse effects on male and female reproductive organs. Further, it is believed that TPP may be corrosive or irritating to the skin.
U.S. Patent Application Publication No. 20080070818 (“the '818 publication”) discloses a lubricating oil composition including at least one sulfurized overbased metal phenate detergent prepared from a C9-C15 alkyl phenol, at least one sulfurized agent, at least one metal and at least one overbasing agent; the detergent including less than 6.0% by combined mass of unsulfurized C1-C15 alkyl phenol and unsulfurized metal salts thereof. Examples A and B disclosed in the '818 publication obtained an overbased detergent having 5.58 and 3.84 mass %, respectively, of unsulfurized alkyl phenol and its unsulfurized calcium salt.
U.S. Patent Application Publication No. 20090143264 (“the '264 publication”) discloses sulfurized metal alkyl phenate compositions having a low alkyl phenol content. The sulfurized metal alkyl phenate compositions of the '264 publication can be prepared by reacting a phenol compound such as tetrapropenyl phenol with an aldehyde to form a phenolic resin and then reacting the phenolic resin simultaneously with a metal base and a first sulfurizing agent.
U.S. Pat. No. 4,328,111 (“the '111 patent”) discloses that overbased phenates, including sulfurized phenates are commonly manufactured in the presence of ethylene glycol which is difficult to remove from the product, thereby wasting raw materials and sometimes leading to undesirable side effects from glycol in the final product. The '111 patent further discloses that in order to remove ethylene glycol, an acidic compound is reacted with a basic compound comprising an overbased metal sulfonate, phenate, or mixtures thereof, and the reaction product is then nitrogen stripped to remove the ethylene glycol.
To reduce any potential health risks to customers and to avoid potential regulatory issues, there is a need to reduce the amount of free unsulfurized alkyl-substituted hydroxyaromatic compound and its metal salt in the salt of a sulfurized alkyl-substituted hydroxyaromatic composition in a simple, cost efficient manner. Accordingly, it is desirable to provide an improved process for preparing a salt of a sulfurized alkyl-substituted hydroxyaromatic composition which has relatively low levels of unsulfurized alkyl-substituted hydroxyaromatic compound and its metal salt.